I am so sorry. Horses are with you for so long that when they go it's like loosing part of your family. The stable where my dd has done 4-H for 15 years (now a project leader) lost 2 horses in a month, both ones she had a close bond with and both rather suddenly and painfully. It is so hard.
What you describe sounds like a crisis that could not have been averted. You gave him every care and close attention. I hope neither you nor your dd blame yourselves.